Gas-Phase Thermal Reactions: Chemical Engineering Kinetics

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AIChE J. The power law kinetics It happens rarely when reaction is elementary in nature. What are elementary and non-elementary reactions? Rate constant k is described as a function of temperature and usually represented by the Arrhenius equation, the above equation will be used with Arrhenius equation and additional parameters Arrhenius parameters are also to be fitted.

To estimate the effect of temperature on the rate of a chemical reaction, the values of Arrhenius parameters are to be known, otherwise experimental data of rate vs temperature can be used to calculate the parameters.

Limiting Cases: Diffusion-Controlled and Activation-Controlled Reactions

The SI unit of the slope is K. A large slope of Arrhenius plot means large value of Ea and vice versa. Arrhenius plot This may be done in PFR by diluting the catalyst bed with inert particles. Each rate equation is fitted with the experimental data and the kinetically and statistically best-fit equation is the rate equation for the experimental data.

Developing a rate equation Fogler, 3rd ed.

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Using power law kinetics for the irreversible reaction, fit the experimental data and find out the kinetic parameters. RUN No. Regression in SigmaPlot Software Including reversibility the following power law form may be used with the experimental data: Where, Ke is reaction equilibrium constant. Only rate equation is changed, the rest of the procedure of fitting the data is the same.

Repeat Example 1 using the power law form shown above.

Use SigmaPlot and compare the results with Example 1. Euler or 4th order RK method can be used for this purpose. Try in Excel using Solver.. Example 2: Fitting the experimental data in a PFR Please see references [10] and [10a]. Determination of equilibrium constant Using Gibbs free energies of formation Equilibrium constant for the dehydrogenation of MCH to toluene using the information of Gibbs free energies of formation from Yaws [10].

The experimental equilibrium constant is given below [11a]. Workout the relationship for mole fraction of each species involved in the reaction in terms of initial mole fractions. Use XI and XII as the fractional conversions for the first and the second chemical reactions, respectively. Homework problem I II Homework problems Hint: Consider ideal gas conditions and avoid fugacity calculations. The previous case is for non-dissociative adsorption. In non-dissociative case, the molecule is adsorbed as a whole and it may be called as molecular adsorption.

In dissociative adsorption, the molecule such as of H2 is dissociated into H-H form and adsorbed. Of course the dissociative case requires 2 sites for the adsorption. Non-dissociative: Dissociative: Dissociative and non-dissociative adsorption It requires some activation energy before the component is adsorbed. Chemisorption is usually activated adsorption. Non-activated adsorption or ordinary adsorption does not depend upon temperature therefore activation energy is zero.

Activated and non-activated adsorption [7a] Hougen and Watson [] developed a similar approach and popularize the Langmuir- Hinshelwood kinetics. Development of a typical kinetic rate equation Homework problem [2] Eley-Rideal kinetics This is useful in the sense that it helps in interpreting the effect of partial pressures of reaction species and even inert and poisons on the rate of reaction. Homework problems C Derive rate expression assuming surface reaction as rate determining step. You may consult Chapter 10 of Fogler [2].

17.5: Kinetics of Reactions in Solution

See also Chapter 2 of Froment et al. Each of the rate equations developed is to be fitted against the experimental data and the one which is the most appropriate kinetically and statistically would be chosen as the best-fit rate equation. Comparing the results to the literature findings and microkinetic analysis may be carried for the final discrimination and for observing the validity of the final rate model.

For more on discrimination among the rival models, see Chapter 10 of Fogler [2], Chapter 2 of Froment et al. Discrimination among the rival models Solve the problem and see the difference for using two different rate equations. What are your comments? Due to the porous nature, the inner surface is many times greater than the outer particle surface.

Now as the reactants have to reach the inner surface through small pores, there may be significant resistance to mass and heat diffusion transfer. Due to the reason, concentration and temperature profiles may be developed across the particle, i. For temperature distribution, if a reaction is exothermic, higher temperatures are possible at the center of the particle than at the outer surface and vice versa for an endothermic reaction.

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Diffusion and reaction for porous catalysts Now as concentration and temperature may vary from point to point so does the rate of the reaction. Therefore, it is required to account for these variations diffusional resistances to define the rate which represents the particle as whole. It may be interpreted that the effectiveness of the catalyst towards the rate of reaction varies within the catalyst particle and this effectiveness has to be considered in the final rate law.

A factor known as effectiveness factor is required.

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Effectiveness factor is defined as the ratio of the observed or overall rate of reaction to the intrinsic rate of reaction at the outer surface of the particle where there are no diffusional limitations. Diffusion and reaction for porous catalysts: Effectiveness factor One such relationship graph is shown in the figure on the next slide. The molecules hitting the wall are momentarily adsorbed and then given off in random directions diffusively reflected.

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The gas flux is reduced by the wall resistance. Knudsen diffusion is not observed in liquids. Bulk diffusion coefficient for gases at low pressure To avoid interpolation, a graphical relationship for collision integral is given on the next slide. The following approach can be used for estimating the mass diffusivity of gases at high pressures. The method is very good for gases at low pressures. At high pressures, due to the lack of large number of experimental data, a comparison is difficult to make.

The properties of gases and liquids. Bulk diffusion coefficient for mixture of gases and gases at high pressure Various design of reactors Reactors for gas phase only [20] You may have a novel idea. How will you define moving bed reactor and fluidized bed reactor? Design of a fixed bed reactor The flow in upward direction may loosen the bed giving rise to attrition and fine particles may increase the pressure drop and may possibly be fluidized. This conforms to a shell and heat exchanger type assembly. The catalyst may be packed inside or outside of the tubes.

Describe the connection issue. SearchWorks Catalog Stanford Libraries. Gas-phase thermal reactions : chemical engineering kinetics. Imprint Boston : Kluwer Academic Publishers, c Physical description xviii, p.

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